Oil recovery by steam stimulation and in situ combustion



June 2, 1970 J, c, ALLEN ET AL 3,515,212

OIL RECOVERY BY STEAM STIMULATION AND IN SITU COMBUSTION Filed Sept. 20, 1968 2 Sheets-Sheet 1 l 71:11. l l Tlcl.. a

J. C. ALLEN ET AL June 2, 1970 OIL RECOVERY BY STEAM STIMULATION AND IN SITU GOMBUSTION Filed Sept. 20, 1958 2 Sheets-Sheet 2 'Tlcl u 3,515,212 OIL RECOVERY BY STEAM STIMULATION AND IN SITU COMBUSTION Joseph C. Allen, Bellaire, Thomas S. Teasdale, Houston,

and George C. Cady, Bakersfield, Tex., assignors to Texaco Inc., New York, N.Y., a corporation of Delaware Filed Sept. 20, 1968, Ser. No. 786,789 (Filed under Rule 47(b) and 35 U.S.C. 118) Int. Cl. EZlb 43/24 U.S. Cl. 166--245 10 Claims ABSTRACT OF THE DISCLOSURE Improved recovery of hydrocarbons from subterranean hydrocarbon-bearing formations is effected by reverse in situ combustion which is initiated spontaneously after the injection of steam sufficient to raise the temperature of the formation to a level at which spontaneous ignition of the hydrocarbons occurs upon injection of a combustion-supporting gas. After completion of this combustion operation, a forward in situ combustion is initiated.

FIELD OF THE INVENTION This invention relates to a method for the recovery of hydrocarbons from a subterranean hydrocarbon-bearing formation by the use of steam, reverse in situ combustion, and forward in situ combustion. More particularly, this invention pertains to a method for the secondary recovery of low-gravity petroleum hydrocarbons, which combines the desirable features of steam stimulation and in situ combustion.

DESCRIPTION OF THE PRIOR ART Steam flooding and in situ combustion are relatively new thermal recovery techniques which potentially can increase greatly the recovery of hydrocarbons from subterranean hydrocarbon-bearing formations.

Steam stimulation has been utilized principally as a thermal stimulation technique which involves the injection of steam into a subterranean hydrocarbon-bearing formation through a production well, following which the well is shut-in for a period of time prior to being returned to production. This technique is referred to as steam soaking or huif and puff and is practiced widely in the production stimulation of low-gravity crudes. It has two main effects, reduction of oil viscosity and well bore cleanup. In general, steam is injected in quantities sucient to heat up the subterranean hydrocarbonbearing formation radially from the production well, the quantities being in the range of 20,000 pounds/hour for periods up to 30 days. Generally, this technique results in a 20- to 50-fold increase when the well is placed on production following the steam injection and soak period. However, the stimulation does not last long, and in addition, the total volume of reservoir heated is not large.

In situ combustion refers to a process in which a portion of the underground oil s burned in place to provide heat at high temperatures. ln this process, a combustionsupporting gas, such as air, is introduced into a well and ignition of a portion of the hydrocarbons in the hydrocarbon-bearing formation is accomplished by any of wellknown methods including downhole heaters, chemical means or spontaneous ignition. The latter method is the most economical. The combustion-supporting gas is injected continuously to maintain a combustion front in the subterranean hydrocarbon-bearing formation `which moves through the formation and drives the formation uids toward a production well or wells from which they United States Patent O ICC are produced. There are two principal variations of the in `situ combustion process, known as forward and reverse combustion. In the former method, a narrow combustion zone is formed and moved radially outward through the subterranean formation from the injection well to the production well. In the latter method, the combustion zone is moved from the production well to the injection well. The reverse method has been utilized for lowgravity crudes since it has the advantage that the formation fluids are displaced into a hot region of the formation and thus are rendered more mobile than in the forward method where the hydrocarbon products have to flow through a cool formation ahead of the combustion front. In practice, in situ combustion may result in 5- to lO-fold production increases. Although stimulation is generally not as great as that realized from steam soaking techniques, the stimulation continues for a longer period of time.

Despite the potential recoveries which may be realized from these two techniques, both have certain inherent problems. In steam flooding, for example, large quantities of extraneous energy have to be supplied, and the cost of this reduces the attractiveness of the method. On the other hand, in situ combustion necessarily consumes a portion of the hydrocarbons in the formation for fuel. Furthermore, adverse permeability distributions in the subterranean formation can cause poor vertical conformance with resultant poor sweep efficiencies. In addition, the application of forward in situ combustion to low API gravity crudes has not succeeded because of the build-up of a bank of viscous hydrocarbons which prevents the ilow of gases through the formation so that adequate combustion-supporting gas cannot be supplied to the combustion front.

Accordingly, it is an object of the present invention to provide an improved process for recovering oil from a subterranean formation containing a low-gravity crude utilizing a combination of steam :and in situ combustion thermal recovery techniques.

Another object is to improve the economics of the process by requiring a relatively short period of steam injection for supplying extraneous energy and utilizing the advantages of spontaneous ignition thereby eliminating additional steps and equipment during ignition of the hydrocarbons in the hydrocarbon-bearing formation.

Still another object is to overcome the difficulty of the build-up of viscous hydrocarbons thereby preventing gas fiow in the reservoir encountered in the in situ combustion of low API gravity crudes, by establishing good gas permeability in the formation.

SUMMARY The invention comprises a method for the recovery of hydrocarbons from a subterranean hydrocarbon-bearing formation by injecting gas, preferably a combustionsupporting gas, followed by steam, establishing a reverse in situ combustion, sustaining the in situ combustion by further injection of the combustion-supporting gas, and following with forward in situ combustion to obtain additional recovery.

DESCRIPTION OF THE PREFERRED EMBODIMENT More particularly, the invention comprises injecting a gas, eg. a combustion-supporting gas such as air, into the subterranean hydrocarbon-bearing formation through an injection well, until said gas is produced through an offset well, thereby establishing good gas permeability in the subterranean formation. The gas injection is then terminated and steam is then injected at 200-800 F. into the formation through the injection well until a high temperature is attained in a zone from 2 to 50 feet radially into the hydrocarbon-bearing. formation adjacent the injection well. Thereafter, the steam injection is terminated and the injection well is placed on production thereby causing the formation uids to flow toward the former injection well, through the heated zone, thus 'becoming more mobile due to viscosity reduction. The combustionsupporting gas, within the formation as a result of the prior gas injection, also moves toward the former injection well. When it comes into contact with the heated hydrocarbons in the formation, spontaneous ignition thereof occurs within the formation and in the periphery of the heated zone where the hot hydrocarbons and combustionsupporting gas are mixed. Thereafter, a combustion-supporting gas is injected into the offset well and a reverse in situ combustion is established, whereby the combustion front moves toward the offset well.

The foregoing process is continued until the combustion front has moved to the vicinity of the offset well. At this time the injection of combustion-supporting gas into the offset well is terminated. Since residual hydrocarbons remain in the formation after the reverse in situ combustion, a forward in situ combustion is then established adjacent the present production well, which is returned to its original function of an injection well, whereby the combustion front is then moved toward the offset well which now serves as a production well. Thus, the formation is again swept by an in situ combustion operation, thereby resulting in additional recovery of the residual hydrocarbons.

The process may be effected in any suitable well pattern including a 5-spot, a 7-spot or similar ring patterns, or it may be applied to parallel rows of wells utilizing one row for injection wells and the neighboring rows for production wells. It may also be applied to a pattern containing a plurality of wells wherein the injection and production steps are applied to sections of the pattern which are scheduled so as to obtain optimum utilization of the process.

In a preferred embodiment of this invention the composition of the combustion-supporting gas is selected to enhance the establishment of an initial gas saturation within the formation and to initiate and support an advantageous spontaneous ignition and combustion of the hydrocarbons in the formation.

hydrocarbons in the formation, which allows much of the gas initially injected to finger through the hydrocarbons and move radially outward through the formation and bypass much of the hydrocarbons. After the steam injection period, the combustion-supporting gas moves back into the heated formation and the exothermic reaction between the combustion-supporting gas and the hydrocarbons generates heat in situ thus establishing the in situ combustion in a reverse ow. Experience has shown that the rate of autooxidation leading to spontaneous ignition is very slow when the ambient formation temperature is less than about 200 F. Consequently, the steam injection is useful to raise the temperature of the reservoir to a level greater than 200 F. where the reaction rate is sufficiently high to achieve spontaneous ignition quickly. Additionally, the steam injection will materially improve the distribution of heat in the formation so that in the subsequent injection of the combustion-supporting gas, a uniform vertical combustion front can be established.

The combustion-supporting gas comprises a mixture of gaseous materials, such as air, which includes at least oxygen. Other components may include one or more other gases such as nitrogen, carbon dioxide, combustion flue gas, and/or a lower hydrocarbon such as methane or ethane. The dissolving of some light hydrocarbon in the formation crude helps to reduce the viscosity of the crude during the initial gas injection step, thereby enhancing ultimate recovery of the oil. Carbon dioxide also has a beneficial solution effect to reduce viscosity.

Steam in the form of high quality (%-j-) saturated steam is the preferred heating fiuid, although superheated steam or low quality saturated steam or hot water also can be used. Steam is preferred for the purpose of transferring heat through the formation because of its advantageous heat capacity and its high latent heat of condensation.

A more complete understanding of the invention may be had `by reference to the accompanying schematic drawing in which FIGS. l to 8 show the various steps of the process as applied to a 5-spot well pattern.

In each of the figures, an injection well 10 is surrounded by offset ring wells 11, 12, 13, 14.

FIG. l illustrates the first step of the method wherein a combustion-supporting gas, such as air, is injected through central well 10l until the air is produced from the offset ring wells 11, 12, 13 and 14, thereby establishing gas permeability in the well pattern. Alternatively, the combustion-supporting gas could be injected through the offset ring wells simultaneously with the injection through the central well.

Also, a period of injection of; (l) steam or some other heating fiuid or (2) combustion-supporting or non-combustion supporting gas and steam or some other heating fluid simultaneously may precede combustion-supporting gas injection to raise the temperature of the formation adjacent to the injected well to induce a better vertical conformance adjacent to the well bore. Also, water or any other suitable fluid may be injected simultaneously with the combustion-supporting gas to induce a better vertical conformance of the gas permeability established in the well pattern.

FIG. 2 illustrates the next step after air is produced from the offset ring wells 11, 12, 13 and 14 by the first step illustrated in FIG. 1. Wells 11, 12, 13` and 14 are either shut-in or produced under back-pressure whereby the pressure against the wall of the wells is regulated, so as to control the rate of pressure decline within the hydrocarbon-bearing formation. Steam is then injected at 20G-800 F. via well 10 until a quantity has been injected which will substantially fill up the pore volume of the hydrocarbon-bearing formation to about a radius of 50 feet around the central well, thereby establishing a hot zone adjacent well 10 which is identified by numeral 15. In the rest of the pattern formation, identified by numeral 16, air saturation will have been established as a result of the first step.

FIG. 3 illustrates the next step of the method wherein steam injection is terminated and central Iwell 10 is produced so as to move hydrocarbons through the formation toward well 10 thereby resaturating the heated formation adjacent central well 10. Furthermore, air movement from the pattern toward central well 10 also will occur, thus resulting in initiating an in situ combustion front designated 18 where the air comes into contact with the periphery of the heated zone 15.

Alternatively, prior to the step illustrated in FIG. 3, the wells could be shut in for a period, whereby the formation would undergo a soak period so as to permit additional distribution of the thermal energy within the formation resulting from the steam injection step.

FIG. 4 illustrates the next step in the method which involves injecting air into the offset ring wells 11, 12, 13, `14 thereby moving the established in situ combustion front 18 outward from central well 10 toward the ring wells, by reverse in sit-u combustion. Formation hydrocarbons are thereby moved through the heated zone 15 to central well 10 where they are produced.

FIG. 5 illustrates the next step in the method wherein the in situ combustion front 18 is allowed to progress to the vicinity of offset ring wells 11, 12, 13 and 14;

thereafter the air injection is terminated at the offset ring wells.

FIG. 6 illustrates the next step in the method wherein air injection into the offset ring wells 11, 12, 13 and 14 is terminated and air is again injected into central Well and initiating a forward in situ combustion front designated 19 in the vicinity of central Well 10.

FIG. 7 illustrates the next step in the method wherein air is continued to be injected into central well -10 causing the combustion front 19 to be moved toward the offset ring wells 11, 12, 13, 14 which are being produced.

FIG. 8 illustrates the final step in the method wherein the in situ combustion is allowed to progress to the offset ring wells |11, 12, 13, 14 and thereafter the air injection is terminated at the central well 10 and the offset ring Wells and central well 10 are produced by pressure depletion of the formation whereby the formation is produced without fluid injection.

Obviously, other modifications and variations of the invention, as hereinafter set forth, may be made without departing from the spirit and scope thereof, and therefore, only such limitations should be imposed as are indicated in the appended claims. For example, in the illustration of the process as applied to a 5-spot pattern, it is possible to reverse the steps as applied to the central Well and to the offset wells.

We claim:

1. A method for producing hydrocarbons from a subterranean hydrocarbon-bearing formation traversed by an injection well and an offset Well, which comprises the steps of:

(a) injecting a gas into said formation via said injection well until said gas reaches said offset well to establish gas permeability in said formation and thereupon terminating injecting gas therethrough;

(b) thence injecting steam through said injection well While said offset well is regulated for a selected period of time to heat said hydrocarbon-hearing formation adjacent said injection well to a temperature sufficient to subsequently ignite spontaneously said hydrocarbons upon injection of a combustionsupporting gas and thereupon terminating the steam injection;

(c) thereafter, injecting said combustion-supporting gas through said offset well while producing hydrocarbons from said injection lwell, igniting said formation by spontaneous ignition at the periphery of the heated hydrocarbon-bearing formation adjacent said injection well and establishing a reverse in situ combustion front thereat;

(d) continuing injecting said combustion-supporting gas through said offset well While producing hydrocarbons via said injection well to cause said reverse in situ combustion front to move away from said injection Well toward and to vicinity of said offset well and thereupon terminating injecting of said combustion-supporting gas into said offset well;

(e) thence injecting said combustion-supporting gas into said injection well and producing hydrocarbons via said offset well, ignitingl said heated formation adjacent said injection well and establishing a forward in situ combustion front thereat; and

(f) continuing injection of said combustion-supporting gas into said injection well while producing hydrocarbons via said offset well to cause said forward in situ combustion front to move from said injection well to said offset well and thereafter, terminating injection of said combustion-supporting gas into said injection well.

2. lIn the method of producing hydrocarbons as defined in claim 1, the additional step of producing hydrocarbons via said injection lwell and said offset well by pressure depletion.

3. In the method of claim 1, the gas used in establishing gas permeability and said combustion-supporting gas being air.

4. In the method of claim 1, step (a) being preceded by the step of injecting a heated fluid into said formation through said injection Well.

5. In the method of claim 1 wherein in step (b) said offset 'well being produced under back pressure.

6. In the method of claim 1, wherein in step (c), said injection of said combustion-supporting gas into said injection well being preceded by a period during which said injection well and said offset well are shut-in.

7. In the method as defined in claim 1, a suitable fluid such as water being injected simultaneously with the combustion-supporting gas to induce a better vertical conformance of the gas permeability established in said formation.

8. In a method as defined in claim 1, said injection Well and said offset well comprising part of an in-line pattern of a plurality of wells.

9. In a method as defined in claim 1, said formation being traversed by a well pattern including a central well and a ring of offset wells.

|10. The method of claim 9 wherein said steps applied to said central well and said steps applied to said ring wells are reversed.

References Cirted UNITED STATES PATENTS 3,174,543 3/1965 Sharp 166-261 X 3,239,405 3/ 1966 Parrish 166-261 3,334,687 8/1967 Parker 166-261 X 3,369,604 2/1968 Black et al. 166-261 JAMES A. LEPPINK, Primary Examiner I. A. CALVERT, Asssitant Examiner U.S. Cl. XR. 166-261 

